The present invention relates to mold structures for compression molding thermoplastics.
Compression molding of glass reinforced thermoplastic sheets is a promising method for producing relatively thin, wide and strong parts such as car hoods, doors and panels. One important prerequisite for the use of glass reinforced composite products in automobile applications is a Class A surface. While there is no universally accepted specification, the Class A surface is a glossy, smooth and polished surface which should be as smooth as that of current automobile exterior parts made from sheet metal.
Current molding processes of glass reinforced thermoplastic composite sheets begins with heating the composite blanks in an oven, typically in infrared or hot air convection ovens. The material is heated above its melting point or if an amorphous material at least substantially above its glass transition temperature. The hot blanks are then pressed between cool mold surfaces (surfaces lower than the melting point or the glass transition temperatures), which are typically 175.degree.-250.degree. F. A molding pressure of one half ton/sq. in. to two tons/sq. in. is applied to the mold during a typical cycle time of 45-60 seconds.
When the composite blanks are heated, they expand (loft) due to the recoil forces within the fibers. The surface of the expanded blanks then cools during its transfer to the mold, resulting in "frozen" resins on the surface. Compression of this blank in the cool mold produces surfaces which are not completely filled with resins, although some hot molten material moves from the inner core to the surface. This creates unfilled areas in the form of exposed fibers and surface porosity or voids. Since the resin at the cold surface is frozen and does not flow, rough boundaries between charged and newly formed areas are also produced. These exposed fibers, porous areas and blank boundaries are the major manifestations of surface roughness, although other physical processes, such as differential thermal shrinkage between fibers and resins, can also result in surface roughness and/or waviness.
Recently it has been found that smooth surfaces can be obtained from neat resin in blow molding by using hot surface molding. The resin is supplied hot to the mold as a parison in blow molding. These techniques, which are based on temperature cycling of mold surfaces using heating and cooling fluids, increase the cycle time of the process and require very complex control schemes which are the major disadvantage of these techniques. These techniques have been demonstrated only for blow molding neat resins. Compression molding of composite sheets is much different from blow molding of neat resins. Heating of composite sheets causes the fibers to loft producing exposed fibers at the surfaces. Attempts to obtain smooth surfaces with composite sheets have involved trying to change the structure of the composite sheets so that the outside layers on the composite sheets have neat resin with barrier layers sometimes being provided to prevent the fibers situated in the middle layers from coming to the surface. These sheets could then be molded using conventional compression molding techniques.
It is an object of the present invention to provide a multilayer composite mold which can be used in current molding systems without major modifications, and can compression mold reinforced thermoplastic composite sheets into finished products that have smooth surfaces, a minimum of exposed fiber, porosity, and blank boundaries.
It is a further object of the present invention to provide a multilayer composite mold which reduces the required molding pressure and therefore reduces press size, which is particularly significant in large part fabrication.
It is a still further object of the present invention to provide a multilayer composite mold which results in short cycle times and therefore increased throughput for each molding operation.